When you are in the creation part of your book, you first need to insert a text block, to do this click on the Text button at the top left of the screen then on add a title or text depending on what you wish.
Then click on the text block to enter your title. A blue frame will appear around your text as well as a toolbox just below.
This toolbox allows you to change the appearance of your text (font, size, color, alignment, bold, italic, etc.). Click on the Text Color button represented by an A, the toolbox will then unfold downwards displaying different color boxes, as well as a color palette.
You can either click on one of the colors already proposed, or choose a custom color with the color palette.